1. Field of the Invention
The present invention relates to an optical switch for use in optical networks. More particularly, the present invention relates to an optical switch incorporating a micromachined adjustable phase hologram capable of quickly and precisely redirecting a light beam.
2. Description of Related Art
Optical networks use light guided in optical fibers to transmit information in the digital domain for voice and data, and in the analog domain for cable television. The ability to transmit large bandwidths over long distances makes optic networks very attractive. WDM (wavelength division multiplexing) further expands the capacity and configurability of optical networks by sending information in one fiber over multiple wavelengths of light, where each wavelength is independently transmitted and detected. Many different switches are available for optical networks.
Manual patch panel products are similar to the manual telephone switchboards used before automatic telephone switches were invented. The drawbacks of patch cords include extremely slow switching speeds and the inability to provide dynamic and programmable routing for reconfiguring the network.
Electronic switches must first convert a light beam into an electrical signal before performing the switching or routing process. The electrical signal must then be converted back into a light signal. These conversion processes add additional costs for electronic converters on each side of the switch. These electrical switches are also necessarily complex, typically requiring a photosensor or other transducer to receive the light signal, as well as a laser diode to re-transmit the electrical signal back into a fiber optic line. The limitations of electronic switches in an optical network include the fact that they can be set up for only one format and their limited data rates.
1.times.3, 2.times.2, or 1.times.N optical switches, also known as bypass or A/B switches, allow an optical network manager to switch to an alternate fiber in the event of a fiber failure or other network problem. Most of these switches are unable to connect any more than two fibers to any other two fibers. One type of optical switch uses mirrors mounted on motors to create a 1.times.N switch, such as those offered by DiCon, which uses a motor-driven apparatus to position a fiber with a target fiber. DiCon claims that a user can build a 16.times.16 matrix by cascading 32 of their 1.times.16 switches. However, this configuration is cumbersome. Furthermore, these switches are very slow, and also have wear and service life issues.
Lithium niobate switches have switching times on the order of microseconds. However, one major drawback of the lithium niobate technology is that it will only support a limited matrix size, at the most eight input fibers by eight output fibers, which is too small for most applications. In addition, the insertion loss of lithium niobate technology is quite high, on the order of 8-12 dB, which means that up to 94 percent of the light is lost when passing through a lithium niobate switch. Lithium niobate switches also have inherently high crosstalk between channels. There are very specific wavelength requirements on the light passing through a lithium niobate switch, which means that WDM cannot be employed with lithium niobate switches. In addition, wavelength specific lasers are more expensive than standard lasers and are more difficult to use. Also, lithium niobate switches are not polarization independent.
Electronically controlled piezoelectric elements are used in optical switches, such as those available from Astarte Fiber Networks. The optical signal from the input fibers is focused into a collimated light beam through a lens. This beam is then automatically directed to the selected output fiber with the receiving lens focusing the light into the receiving fiber core. The light beams are routed using electronically-controlled piezoelectric elements. Signals are switched in approximately 150 milliseconds by changing the direction of the light beam from the input fiber to a different receiving output fiber.
Electro-optic polymer waveguide technology has been used by Lightwave Microsystems to make an aggregation of 2.times.2 cross-point switches based on planar waveguides to build optical switches in 1.times.4, 4.times.4, 1.times.8, and 8.times.8 configurations.
Micromirror devices such as those developed by Texas Instruments use a bed of microscopic tilting mirrors. They can also only resolve a few spots, and have switching speeds on the order of 20 .mu.sec.
AT&T is proposing to use a N.times.N matrix of pop-up mirrors to build a N.times.N switch. These pop-up mirrors raise up into the path of the light beam to route it. Such a switch is likely to have reliability problems. Such a switch has a switching speed on the order of tens of milliseconds.
What is needed is an optical switch which deals directly with optical signals, has fast switching speeds, is capable of routing WDM input signals, is polarization independent, is scalable to large switches, and avoids the complexity of other switches.